An Audio Conferencing licence enables you to provide a dial-in phone number for Teams meeting attendees, and to dial-out during a meeting to invite other callers into that meeting.
Currently there are two promotions that entitle EA and CSP customers to free Audio Conferencing licences (https://bit.ly/33NIFkN), with both promotions running until 31 December, 2021 (https://bit.ly/3gn1wu8).
However, Microsoft have now announced that over the coming months they will add unlimited dial-in capabilities for Teams meetings for existing Office 365 and Microsoft 365 licences. Find the information towards the bottom of this page: https://bit.ly/3kdqvBg.